No, you haven't. You're having a bad day. It's never a straightforward path - there are sometimes steps back. As long as your overall progress is forward, that's what counts. If it's been going on for longer than a day or two, then maybe it's worth going back to your GP?
I've been off anti-ds for some years now, but there are still days where everything is very bleak and hopeless. They don't happen as frequently as they once did, and they don't last so long, but I've sort of accepted that I'll probably never be entirely free of it. I can live with mostly, though.
Do you think the light affects you? I've certainly noticed the changing season. If it does, you could try a SAD lamp. Also, as I've got older, I find I'm much more affected by hormones, and if I realise feeling almost suicidal matches my cycle, then it's already easier to deal with, because I know it will pass in a day or two. It may not be relevant to you, but worth considering.
You haven't failed. You've got through this before, and it shows you've got the strength to do it again. Just one day, one hour, one minute at a time.